1. Field of the Invention
The present invention relates to a method whereby the movements of an object are recorded in an image. A primary application of the invention is in the medical field in which it is sought to represent heart motion, lung motion or blood flow or on the contrary to remove from the image any disturbances resulting from motion artifacts. The invention will be described in the case of a nuclear magnetic resonance (NMR) imaging application without being thereby specifically limited to this application. Similarly, the invention will be studied in the case of cyclic movements to which it can readily be applied. However, it can also be adapted to movements which occur only once.
2. Description of the Prior Art
In conventional practice, the representation of moving objects involves acquisition of successive images (the term "image" being clearly used in a broad sense) uniformly spaced in time during the movement to be studied. By displaying these images at a low rate, the movement of the object can be closely studied. By means of cameras, modern methods of television permit the application of this technique on condition that the cameras are sufficiently fast to acquire images which are very close to each other in time and also on condition that these images are subsequently displayed by slowing-down the projection rate. Unfortunately, these general techniques are not readily applicable to methods of imaging by nuclear magnetic resonance (NMR) or even to methods of representation of tomodensitometric cross-sections obtained with tomodensitometers.
In fact, the acquisition of tomographic images with instruments of this type is obtained only after a treatment involving reconstruction of the cross-section images. In the NMR technique, for example, the body particles located in the cross-section of a patient's body to be studied are subjected to an excitation sequence during experimentation and restitute at the output a signal containing an item of information which is representative of their nature. Unfortunately, all the particles of the cross-section re-emit a signal at the same time. With the known methods of image reconstruction, the difficulty of this simultaneous response is circumvented by reiterating the experiments a certain number of times and by changing the conditions of acquisition of the signal from one experiment to another. Typically in NMR imaging, 256 successive experiments are performed over a total period having a minimum duration in the vicinity of half a minute. In the field of tomodensitometric images, the total time of acquisition of all the views (frames) is of the order of one second. Now in one case as in the other, the cycles of fluctuation of the human body, for example in the region of the heart, are of the same order. In consequence, the total number of acquired sequences or the total number of detected views is obtained during a period in the vicinity of that of the heartbeat. Thus the images are fuzzy in the immediate region of the heart.
In order to overcome this disadvantage of motion, an initial solution consisted in carrying out a synchronization. However, and particularly in NMR, the most rapid methods of excitation known as steadystate free precession (SSFP) and corresponding to free precession in a state of balance of magnetic moments of the particles must be applied to a natural frequency. This natural frequency depends on the spin-spin relaxation time (designated as T.sub.2) of the magnetic moments of the particles to be studied. It is apparent that this natural frequency opposes synchronization. There is in fact no reason why this natural frequency should be a multiple of the heart rate. If synchronization is adopted (and consequently if the idea of the SSFP mode is abandoned) and if it is desired in addition to produce an image which is said to be in T.sub.2 (in order to show the spin-spin relaxation times), it is in that case necessary to wait between each experiment for a period of time equal to three or four times the interval T.sub.1 (spin-lattice relaxation time) which is characteristic of the particles under study. In other words, the duration of each experiment is of the order of two seconds. The acquisition of the 256 sequences then leads to a time-duration of about 10 minutes for a single synchronized image during the cycle, taking into account the losses of time during synchronizations. However, physicians require about 32 images for a complete cardiac cycle in order to be able to gain a clear understanding of functioning of the heart. This results in a total examination time of 5 hours. A period of this length, however, is quite intolerable for patients. Furthermore, even assuming that patients could tolerate such a long period of examination, the cardiac cycle is not constant. The result is that synchronization of the other instants of the cycle with respect to a precise date of the cycle does not always correspond to the same state of the heart. A phase lead or phase lag with respect to a predetermined state of the heart depends on the acceleration or slowing-down of the heart rate with respect to a nominal estimated heart rate. As a result, the images exhibit artifacts.
In an alternative mode of excitation known as fast T.sub.2 with limited flip-over and with synchronization, it is possible to achieve a much higher speed since typically a fast T.sub.2 cycle lasts approximately 50 ms. In consequence, 32 phases of the cardiac cycle can be acquired over a period substantially equal to twice this cycle. In imaging with normal resolution (256 sequences per phase), this results in a total experiment time of 10 minutes. However, fast T.sub.2 imaging cannot be carried out simultaneously in a single slice since the NMR signal is excessively attenuated and there is a loss of contrast. There is accordingly employed a multislice technique so as to perform measurements in other cross-sections during dead periods. In practice, the experiment then lasts four times as long, namely 40 minutes, which is also quite excessive. Furthermore, there again arises the same problem of synchronization as that mentioned above. When the heart rate is not constant (over a period of 40 minutes?), the images also exhibit artifacts.
Another technique has been conceived by Michael L. Wood and R. Marc Henkelman (of the Ontario Cancer Institute and Department of Medical Biophysics, University of Toronto, Toronto, Canada). This technique has been published in Medical Physics, Volume 13, No. 6 of November/December, 1986. It essentially consists in determining an average value over a number of successive images in order to eliminate motion artifacts. This imaging technique, which is concerned with elimination of effects of motion of moving parts, is clearly not capable of restituting the moving image of the motion to be recorded. In addition, this technique is not totally effective and certain parasites may remain in the image.
The solution proposed by the invention is a general solution which makes it possible according to requirements to represent the movements of a moving object in slow motion which produces the desired fine-detail resolution without resulting in a corresponding multiplication of acquisition times. Furthermore, in cases in which moving parts are considered as disturbances, the method according to the invention provides an image of the stationary portions of the object which are free from components relating to motion. It is essentially considered in accordance with the invention that the movement of an object is analyzed in an image as a time variation in luminosity of the image elements or so-called pixels of said image. And it is suggested that this time variation in luminosity of a pixel can be replaced by a spectrum of frequency components (of a basic elementary signal when the movement studied is cyclic). The spectral resolution is preferably the so-called Fourier resolution. The time signal of variation in luminosity of each pixel is accordingly replaced in the case of each pixel by the Fourier transform of said luminosity.
In a simple case, when the phenomenon under study is cyclic, it can be considered that the luminosities of the pixels have a continuous component, a fundamental component of cyclic variation as well as harmonic components of cyclic variation. Assuming that the cycle is the same throughout the body under study, resolution of body motion into a Fourier series makes it possible in successive pixels to obtain a series of images which are representative in each case of the fixed image, of the image of variation at the fundamental rate, and of the images of variation at the frequency of the harmonics of the fundamental rate. The total image of motion can then be reobtained by employing as many image generators plus one as there are image harmonics to be recorded. The participations of each harmonic image is combined, pixel by pixel, and the image thus combined is then displayed. In order to obtain slow motion at a speed as low as requirements may dictate, it is only necessary to produce a displacement of the image generators as slowly as may be desired. In fact, a harmonic analysis of the image is thus performed. It should be noted that an image of the moving parts alone can also be obtained by eliminating the continuous image component and that, on the other hand, an image which is free from any motion artifact can be obtained by employing this image of a continuous image component alone.